1. Field of the Invention
The present invention relates to a sound reproduction device and a portable terminal apparatus including the acoustic reproduction device. More particularly, the present invention relates to a sound reproduction device having a capability of acoustically driving a panel with sound pressure, and a portable terminal apparatus including the same acoustic reproduction device.
2. Description of the Background Art
Conventionally, it is considered to be important to reduce dimensions or thickness of an electronic apparatus, in particular, a portable apparatus. For example, it is conceivable to integrate two devices in an electronic apparatus, which differ in their functions, into one device, and thereby to reduce the entire size of the apparatus. Described next is a case where a sound reproduction capability is added to an apparatus having a specific capability in order to achieve a size reduction, typically a case where the sound reproduction capability is added to a display device.
Conventionally, there have been techniques devised for integrating a display device and a sound reproduction device together. FIG. 12 is a perspective view showing a conventional display device having a sound reproduction capability. In FIG. 12, a display device 1 includes a display panel 2, an organic electroluminescent (EL) panel 3, a film speaker 4, and electrical signal cords 5 and 6. The display panel 2, the organic EL panel 3, and the film speaker 4 are integrated together by an adhesive or the like. Hereinbelow, an operation of the display device 1 is described.
The display panel 2 has characters or the like printed thereon. The display panel 2 is illuminated from behind by the organic EL panel 3 to which voltage is applied via the electrical signal cord 5, so that the characters or the like printed on the display panel 2 are displayed. The film speaker 4 is made of, for example, a resin film having a piezoelectric effect, and caused to vibrate when an acoustic signal is applied thereto via the electrical signal cord 6, so that the display panel 2 and the organic EL panel 3, which are integrated together with the film speaker 4, are caused to mechanically vibrate, thereby generating sound. The above-described display device 1 is capable of making the user feel as if the sound is being generated by a display screen.
There is another conventional technique which makes the user feel as if the sound is being generated by the display screen. Specifically, in this conventional technique, reproduced sound is outputted from a loudspeaker provided on the front of the display device. FIG. 13 is a cross-sectional view of a conventional display device in which reproduced sound is outputted from a loudspeaker provided on the front thereof. In FIG. 13, a display device 10 includes a transparent diaphragm 11, a plurality of transparent electrodes 12 formed on both sides of the transparent diaphragm 11, a support 13, and a cathode ray tube (CRT) 14. Hereinbelow, an operation of the display device 10 is described.
Each transparent electrode 12 is connected to a cord (not shown) via which an electrical signal is applied thereto. The transparent diaphragm 11 is made of a polymeric piezoelectric sheet material, and caused to vibrate by an electrical signal supplied from the plurality of transparent electrodes 12, thereby generating sound. The transparent diaphragm 11 and the plurality of transparent electrodes 12 are made of transparent materials, and therefore an image on the cathode ray tube can be seen therethrough, whereby it makes the user feel as if the sound is being generated by a cathode ray tube screen.
In the above-described display device 1, three elements, i.e., the display panel 2, the organic EL panel 3, and the film speaker 4, are integrated together, and therefore the film speaker 4 is caused to vibrate together with the display panel 2 and the organic EL panel 3, thereby reproducing sound. However, a diaphragm formed by the three elements integrated together is heavy in weight, resulting in reduction of a pressure level of sound to be reproduced. Moreover, an integral structure of the three elements increases the stiffness of the diaphragm, and therefore the diaphragm has difficulty in vibrating at low frequencies, resulting in reduction of low-frequency sound pressure levels.
In the above-described display device 10, the screen of the cathode ray tube 14 is entirely covered by the transparent diaphragm 11, and a space between the cathode ray tube 14 and the transparent diaphragm 11 is kept small in order to save space. Accordingly, an acoustic compliance of the space is considerably small, making it difficult for the display device 10 to perform satisfactory low-frequency sound reproduction.
As described above, conventionally, in the case where a display device and a sound reproduction device are integrated together, the sound reproduction device has difficulty in performing satisfactory sound reproduction (in particular, satisfactory low-frequency sound reproduction). This is not restricted to the display device, and a similar problem can be caused to a device having a different capability if a sound reproduction capability is additionally added thereto. It is conceivable that if the film speaker is provided to add the sound reproduction capability to the device having a different capability, the capability of the film speaker is reduced due to the mass and stiffness of the device.